Occasionally lowest toilet in the house will double flush with a single press of the handle. It will release the water from the tank into the bowl, which then swirls down the drain and the e bowl refills. At this point normally the bowl is full and the tank refills. On occasion, the bowl will refill and then swirl and empty again before the tank fills completely. The tank does not seem to take any longer to fill. Any thoughts? The toilet tends to plug up easily also occasionally. Usually a large bubble comes up from the drain and all is well. This only happens with this toilet and only 4 or so times a year. The toilet is used daily.

I think your vent is clogged and that allows the drain water from the first flush to siphon the second fill. Vent stacks should be rodded from the roof to the basement floor plus 20'.